Site hosted by Angelfire.com: Build your free website today!

 

 

 

Dry Dock Propeller Work

The following are the sections that we will be covering on propeller work in a dry dock:

Major Components of a Propeller

Propeller Problems

Modifications

Checks to be carried out if the ship’s propeller comes in contact with an underwater object at high speeds:

Checks to be made prior to undocking

Documentation that must be provided to the Docking Officer:

Propeller Removal and Installation

Unseating the Propeller

Seating the Propeller

Propeller Flushing & Preserving

Tail Shaft

 

 

Major Components

The major components of a propeller are:

Propeller Cap
The propeller cap, also called the fairwater cap or dunce cap, fits over the end of the propeller shaft and propeller nut (boss nut) and is bolted to the after face of the propeller hub. The cap's conical shape completes the hydrodynamic contour of the hub and protects the threaded shaft end and the propeller nut from damage by contact with seawater. The cap is filled with preservative to prevent seawater from contacting the ferrous shaft and propeller nut. Propeller caps may be one- or two-piece designs. Ships equipped with Prairie air systems can have two-piece propeller caps with Prairie air system components housed in the forward section.

Cover Plates
Cover plates are preformed, semicircular metal plates installed over the juncture between the propeller hub and the propeller cap and between the forward and after sections of a two-piece propeller cap. Cover plates are attached with flat-head screws. When installed, cover plates maintain the contour of the surface of the propeller assembly and prevent fouling by line, wire, or other foreign objects. Cover plates are frequently referred to as fairwater covers, fairwater plates, or fairing plates. On some propellers the juncture is faired with cement grouting material.

Rope Guards
Rope guards are fairwaters attached to the strut bearing housing to protect the area between the forward face of the propeller hub and the strut bearing. Rope guards may be constructed of fiberglass or metal. Fiberglass rope guards are attached with flat-head machine screws. Metal rope guards may be welded in place or they may be attached with flat-head machine screws. Unlike the cover plates attached to the propeller cap, rope guards are stationary and do not rotate with the propeller.

 

 

 

Propeller Problems

Propellers should be maintained in top condition at all times. The main factors that detract from optimum condition are fouling, cavitations and physical damage. Any distortion from their true shape can cause an imbalance and hence vibration, which in turn causes increased cavitations, loss of thrust, drive shaft damage, wear on the numerous bearings, and increased fuel use due to decreased efficiency.

Cleaning

The simplest form of maintenance on a propeller is to clean it. Even a 1mm layer of accumulated fouling or calcium deposits on a propeller will significantly increase its roughness, and within 12 months or so can increase an ISO class I to an ISO class II, or a class II to a III. This causes large increases in fuel consumption. Practical figures and elaborate tests indicate a 6 to 12% gain in fuel consumption in polishing a propeller from a class III condition to a class I condition. Some propellers support marine growth up to 20 mm thick, which obviously has a major effect.

Cleaning, or polishing, involves the removal of this growth, leaving behind a clean and highly polished surface. The manner in which this is done is important. If the growth is removed rapidly using harsh abrasives, the surface of the blades may look shiny, but they will be deeply scratched. These scratches will themselves impart a roughness that is beyond the 1.6 micron (CLA) Ra* tolerance required for a class I propeller. Additionally, the scratches will provide an ideal key for further calcification, micro- and macro-fouling, speeding up the fouling process and often making things worse than they were before. The correct procedure for polishing a blade leaves a very smooth surface, which will resist future fouling. Although slightly more time consuming to achieve, this level of finish is desirable as its long-term effects on fouling and engine wear more than compensate for the initial costs.

When to polish?

A propeller polished to a 1m finish will maintain it's efficiency for approximately 9 months, however, depending on the type of vessel and it's trading patterns, this may be reduced. For example, vessels with longer layover periods will incur a higher fouling penalty, as the propeller is stationary. Vessels trading constantly will have less of a fouling problem. It has been proven propellers polished to a 1m finish still being close to optimum conditions after 10 months and more.

Damage repairs

Propellers can easily be damaged if they strike a buoy, or hit floating debris or ice. The damage is usually to the tip, which can become bent or which can even have large chunks of metal taken out of it. Physical damage of this nature is what most commonly causes vibrations. The solution in this case is to trim the blades equally to remove the damage and achieve a proper balance, and reduce excessive cavitations. The techniques used in this eventuality rely on extremely accurate measurement of the diameters of the individual blades to reduce to a minimum the amount of material removed, but also to ensure a perfect balance. Bad trimming can result in an even worse problem, and cases are on record where inaccurate trimming has resulted in propellers being damaged beyond repair, or even destroyed completely.

 

 

Modifications

There are 3 main types of modifications:

1) Diameter reduction - Easily and inexpensively performed under water, this is the usual method for increasing rpm and balancing the ratio. The blade tips are cropped and faired.

2) Pitch reduction - This involves twisting the blades and can only be accurately done in a workshop as the blades need to be heated to prevent cracking. The twisting usually distorts the hub, which requires machining to ensure a proper re-fitting to the shaft. Although more expensive, this is one of the most effective modifications as there is no loss of blade material. It is ideally suited to blades smaller than 4,000mm dia.

3) Trailing edge modification - This is achieved by either bending the trailing edges, or by cutting them. Both operations can be performed in-water and can achieve an effect on the rpm of approximately 5%. In bending, the trailing edge is slightly modified, without loss of material or strength of the blade. With cutting, a small amount of trailing edge material is removed and the edge re-ground. Templates are used to ensure accuracy, and the effects can be calculated to within approximately 1% rpm accuracy.


Checks to be carried out if the ship’s propeller comes in contact with an underwater object at high speeds:

The stern tubes, Plummer blocks, bulkhead glands, thrust blocks and gear cases shall be examined for cracks and distortion. The propellers & shaft sections shall be checked for bending. Shafts shall be disconnected as necessary to enable readings to be taken. The shaft alignment, alignment of gearing to shafting and of turbines to gearing, shall be checked. The main gearwheel rim and teeth and associated pinion teeth shall be checked for distortion.

In the event of severe damage to the final reduction gear, the remaining gear meshes and gearbox internal couplings shall be examined. Propeller shaft flexible output coupling, shall be examined for damage to flexing elements and bearing and sliding surfaces, and for distortion and cracking of main components. Where the main propulsion machinery is mounted on a raft, the alignment of the engines to the main gearing shall be checked.

 


Checks to be made prior to undocking

Ensure that all work is complete prior to commencing undocking procedures
A memo must be prepared and distributed to the CO, XO etc. to ensure that every one concerned is informed of their duties.
Inspect the dock with senior ET, and HT, CFTSD staff and Docking Officer, openings in the ship's bottom are clear, all gratings, rope guards and eddy plates are properly secured, shafting and rudder are not obstructed.
All sea connections are shut.
Verify the weight log certificate (no heavy weights have been shifted)
Ensure ship's tanks are at the same level as they were when the ship entered the dock
Meet with all concerned to discuss flooding procedures
Propellers are on the dock marks
Cathodic protection anodes, both sacrificial and impressed current are free of paint and masking material
All openings in the hull associated with hull outfits and sonar domes are shut before flooding commences
Debris screens or canvas are fitted over sonar domes to prevent damage
Rudder is locked in mid-ship position
Report to CO that ship is ready to undock, sign undocking certificate as required

Documentation that must be provided to the Docking Officer:

Undocking Certificate

The CO will provide the Docking Officer with the certificate in duplicate, this certificate will state that:

• All sea connections, underwater valves and fittings are in efficient working order.
• All sea valves and other underwater work have been examined and are shut and watertight and will be examined for water tightness when there is sufficient water to do so.
• Personnel are stationed in each space containing sea valves to observe water tightness and communication from manned are established.
• All additions, removals or shifts of weight (solid or liquid) have been recorded and reported to the Docking Officer.
• All tank and compartment soundings have been accurately checked within 24 hours of the planned docking.
• The cathodic protection continuity has been tested and certified acceptable.
• When grounding plates are fitted the D.C. resistance between the immediate internal connections of each plate does not exceed 0.01 ohms.
• The ship is considered to be in all respects ready for undocking

 

 

All Rights Reserved ® Copyright © 2004
www.angelfire.com/rnb/drydocking/
E-mail Webmaster